1. Klára Ambrožová1*, Kamil Láska1
An objective circulation pattern classification for the Antarctic Peninsula Region
Klára Ambrožová1*, Kamil Láska1
1Department of Geography, Faculty of Science, Masaryk University, Kotlářšká 2, Brno, Czech Republic
*Contact:
The Antarctic Peninsula (AP) region experienced intensive warming during the second half of 20th century, while a pronounced coolling spreads along the region since 2004 (Turner et al., 2016). An objective circulation pattern classification, which has been presented for the first time for this area, will be a useful tool to study the relationship between atmospheric circulation and air temperature variation as well as recent glacier mass balance development in the north-eastern part of the Antarctic Peninsula (Láska et al, 2017).
The classification has been created using the self-organizing map (SOM) algorithm (Kohonen, 2001) implemented in the MATLAB software.The input data was the daily mean sea level pressure from the area 48°–81°S, 30–90°W, which comes from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis from 2005–2015 (4017 days). The sea level pressure for a specific point was removed from the analysis if the elevation of that point was above 500 m a. s. l., which enlarged the reliability of the sea level pressure fields (Nigro et al., 2011). The sea level pressure anomalies were acquired by subtracting domain-averaged sea level pressure from the actual value at each grid point. For this contribution, the classification consisting of 3x5 nodes is presented.
Antarctic Digital Database, 2016 (adjusted).
Statistical indices and Sammon map
Circulation pattern classification for the AP Region
From the measures calculated for different classifications, it is apparent that the within-type standard deviation (WSD) rose continuously with increasing number of nodes, while the explained variation (EV) increased more intensively at the beginning and it flattened out from the 4x5 nodes classification on.
From the Sammon map for the 3x5 nodes classification, it is clear that the circulation pattern [1,1] differs distinctly from all the others. On the other hand, the circulation patterns [1,2] and [1,3] are very close to each other, as well as patterns [1,5] and [2,5].
Occurence of circulation patterns
During the period 2005–2015, the most frequent circulation pattern was [1,1], which comprised of more than 10 % of the days. More than 8 % of days were also classified as patterns [1,3] and [1,5]. The least frequent circulation patterns were [2,3] and [2,4], representing a combination of zonal pattern with cyclones on both sides of the Antarctic Peninsula.
AWS3 in Petuniabukta ( © Kamil Laska)
The most frequent circulation pattern [1,1] was observed rather in autumn and winter, while in spring, the circulation pattern [1,5] reached the maximum relative frequency of 12 %. In summer, the occurence of the distinguished circulation patterns was the most evenly split.
While in case of some circulation patterns, the relative frequency does not differ by more than 5 % in all the years (e.g. [2,4] or [3,5]), in other cases (for instance [1,1], [2,1], [1,5] or [2,5]) the occurence is very variable. The relative frequency of [2,4] rose quite steadily during the study period.
Summary
15 circulation patterns for the Antarctic Peninsula region were distinguished based on self-organizing maps algoritm with the use of sea level pressure data from ERA-Interim database.
During the period 2005–2015, the most frequent was the circulation pattern [1,1], which represents a deep cyclone west to the Antarctic Peninsula and a high pressure ridge in the Weddell Sea. This circulation pattern corresponds to the Amundsen-Belingshausen Seas Low and was almost twice as frequent in autumn than in spring. Its occurence also varied remarkably during the study period.
Within 6 circulation patterns ([1,4] – [3,5]), pronounced zonal flow around 50° S and one or more cyclonic systems in the Belingshausen or Weddell Seas were apparent. Although the occurence of these circulation patterns was relatively low during 2005–2015, if considered together, these patterns happened in 36 % of the study period.
Self-organizing map of sea level pressure anomalies over the Antarctic Peninsula. For the purpose of visualization, the fields were interpolated with the use of Radial Basis Functions.
In the presented results, 15 different circulation patterns were distinguished. It is apparent that moving from left to right in the first row, the circulation pattern changes from a deep cyclone in the Bellingshausen Sea ([1,1]) via a high pressure ridge extending from the Antarctic interior ([2,1]) to a low pressure area in the northern Weddell Sea ([3,1]).
Moving from the upper to the lower row, it is clear that the pattern in the sea level pressure becomes more zonal, which is the most pronounced in the circulation pattern ([5,3]).
References
Acknowledgements
TURNER, J. et al. (2016): Absence of 21st century warming on Antarctic Peninsula consistent with natural variability. Nature, 535 (7612), p. 411–415.
LÁSKA, K. et al. (2017): Response of glacier mass on recent temperature cooling in northeastern Antarctic Peninsula. Geophysical Research Abstract. 19, EGU2017–2601.
KOHONEN, T. (2001): Self-Organizing Maps. Berlin: Springer, 501 p.
NIGRO , M. et al. (2011): A Weather-Pattern-Based Approach to Evaluate the Antarctic Mesoscale Prediction System (AMPS) Forecasts: Comparison to Automatic Weather Station Observations. Weather and Forecasting, 26 (2), p. 184–198.
The authors would like to thank the members of the Czech Antarctic expeditions 2012–2017 for their field assistance. This research was supported by the Project of Masaryk University MUNI/A/13/2015, the Czech Science Foundation (project GC J) and the project Czech Polar Research Infrastructure LM funded by the Ministry of Education, Youth and Sports of the Czech Republic.